Telephone numbers in the United States, Canada, and the Caribbean consist of 10 digits divided into three components: The first three digits are the area code; the next three digits are the prefix or exchange; and the final four digits are the suffix, or local number. For each area code-prefix combination, the 10,000 possible numbers for a suffix can be subdivided into banks or blocks of consecutive numbers: 1000-banks (Nnnn), 100-banks (NNnn), or 10-banks (NNNn).
As the sampling method of random-digit dialing (RDD) evolved, the underlying assumption was that residential telephone numbers tended to cluster. Based on this assumption, one of the earliest methodologies consisted of simply adding one to a directory-listed number or randomizing the last digit of a directory-listed number. Both methodologies proved to ...
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